A balancing apparatus for compensation of unbalance is shown in German Application No. DE-OS 23 57 629 which corresponds to U.S. Pat. No. 3,950,897 (incorporated by reference herein). The present inventive method is used to control this device. With this method, a grinding wheel is rotated at a constant number of revolutions per minute. However, in present applications, constant cutting speed and consequently constant surface speed of the machining rotor are required for grinding wheels and similar rotors used to machine a workpiece. As the workpiece is being machined, there is obviously wear on the peripheral surface of the rotor, in particular, the grinding wheel, so that the number of revolutions per minute of the rotor has to be increased. In general, the speed range is in excess of 750 to 2000 revs per minute. Hence, unavoidably, the rotor operates above and below a resonance frequency, i.e., passes through a critical speed range as it goes from subcritical to overcritical due to the given mass of bearings, spindle and rotor, e.g., headstock and grinding wheel. As the critical speed range is passed through, there is a phase shift from 0 to 180 degrees between the angular location of unbalance and the respective vibratory motion. Consequently, the vibration signal supplied by the transducer can no longer be used for accurate control of mass corrections as the phase shift will always occur when the critical speed range is exceeded.
Furthermore, the unbalance signal supplied by the transducer depends on the unknown machine behavior, which varies constantly because, in machining operations involving rotors, such as grinding wheels, the rotor mass is constantly changing. On the other hand, continuous balancing of the rotor is needed during the machining operation in order to obtain good results and avoid poor quality, chatter, waviness and unwanted relative motion between the workpiece and the tool. The present invention is directed toward filling that need.